A Passive optical network (PON) has been proposed as a very promising solution to a broadband optical access network. A variety of PON solutions have been proposed in recent years, such as TDM-PON, WDM-PON, OCDM/OFDM-PON and so on. Particularly, the TDM-PON technology based EPON and 10 GPON have been standardized and are currently being deployed in many countries, and these solutions can offer a data transmission rate up to 10 Gbits/s, but there are rapidly developing Internet services and constantly increasing bandwidth demands, and it is desirable from the perspective of a long term to define a next-generation PON (NGPON) access system capable of being compatible with the current PON system while offering a bandwidth far above 10 Gbits/s.
While the OCDM/OFDM-PON is still in its infancy, the WDM-PON is a relatively mature alternative solution capable of a data rate above 40 Gbits/s. In the WDM-PON, each ONU is allocated with a dedicated wavelength, and the WDM-PON has numerous advantages of a high capacity, compatibility with the legacy PON, etc. With a number of stacked wavelengths, the total capacity per feeder fiber can easily exceed 40 Gbits/s and even reach 100 Gbits/s.
However, besides the need to satisfy a bandwidth demand for a downstream/upstream signal between an Optical Line Terminal (OLT) and each Optical Network Unit (ONU) in WDM-PON, intercommunication between different ONUs has become essential because a user can thereby share data with another ONU at a very high speed and a low delay. In an example of such an application scenario, universities and enterprises need to communicate a high amount of data between their different campuses or branches or different base stations need to cooperatively operate with each other. However in conventional WDM-PON architecture, direct intercommunication is not possible between the different optical network units, because only upstream and downstream transmission links between the optical line terminal and each optical network unit are available, thus greatly limiting the flexibility and efficiency of the network.
In order to enable communication between the different optical network units, there are conventional solutions as illustrated in FIG. 1(a) and FIG. 1(b). In FIG. 1(a), the different optical network units are fiber-connected, and this enables direct communication between the different optical network units, but a large amount of wiring means will induce both a high wiring cost and troublesome network maintenance. Moreover, there is another solution as illustrated in FIG. 1(b) where different optical network units communicate with each other over a communication link of the optical network units to the optical line terminal via a remote node, but such communication has to undergo two conversions of optical to electronic to optical (O-E-O), and moreover, there is generally a large distance, typically tens of kilometers, between the optical network units and the optical line terminal, so both such a transmission distance and the optical to electronic to optical conversion process will necessarily incur an extra delay in communication between the optical network units, and will also add to an effort of processing at the optical line terminal and increase the complexity of the system.